This invention relates to field electron emission materials, and devices using such materials.
In classical field electron emission, a high electric field of, for example, .apprxeq.3.times.10.sup.9 V m.sup.-1 at the surface of a material reduces the thickness of the surface potential barrier to a point at which electrons can leave the material by quantum mechanical tunnelling. The necessary conditions can be realised using atomically sharp points to concentrate the macroscopic electric field. The field electron emission current can be further increased by using a surface with a low work function. The metrics of field electron emission are described by the well known Fowler-Nordheim equation.
There is considerable prior art relating to tip based emitters, which term describes electron emitters and emitting arrays which utilise field electron emission from sharp points (tips). The main objective of workers in the art has been to place an electrode with an aperture (the gate) less than 1 .mu.m away from each single emitting tip, so that the required high fields can by achieved using applied potentials of 100V or less--these emitters are termed gated arrays. The first practical realisation of this was described by C A Spindt, working at Stanford Research Institute in California (J. Appl. Phys. 39(7), 3504-3505, 1968). Spindt's arrays used molybdenum emitting tips which were produced, using a self masking technique, by vacuum evaporation of metal into cylindrical depressions in a SiO.sub.2 layer on a Si substrate.
In the 1970s, an alternative approach to produce similar structures was the use of directionally solidified eutectic alloys (DSE). DSE alloys have one phase in the form of aligned fibres in a matrix of the other. The matrix can be etched back leaving the fibres protruding. After etching, a gate structure is produced by sequential vacuum evaporation of insulating and conducting layers. The build up of evaporated material on the tips acts as a mask, leaving an annular gap around a protruding fibre.
A further discussion of the prior art is now made with reference to FIGS. 1 and 2 of the accompanying diagrammatic drawings, in which FIG. 1 shows basic components of one field electron emission display, and FIG. 2 shows the conceptual arrangement of another field electron emission display.
An important approach is the creation of gated arrays using silicon micro-engineering. Field electron emission displays utilising this technology are being manufactured at the present time, with interest by many organisations world-wide. FIG. 1 shows basic components of such a display in which a field electron emission current is extracted from points 1 by applying a positive potential to gate electrodes 2. The extracted electrons are accelerated by a higher positive potential to a patterned phosphor on conducting strips 3 on a front plate. Pixels are addressed by energising horizontal and vertical stripes in a crossbar arrangement. The device is sealed around the perimeter and evacuated.
A major problem with all point based emitting systems is their vulnerability to damage by ion bombardment, ohmic heating at high currents and the catastrophic damage produced by electrical breakdown in the device. Making large area devices is both difficult and costly.
In about 1985, it was discovered that thin films of diamond could be grown on heated substrates from a hydrogen-methane atmosphere, to provide broad area field emitters.
In 1991, it was reported by Wang et al (Electron. Lett., 1991, 27, pp 1459-1461) that field electron emission current could be obtained from broad area diamond films with electric fields as low as 3 MV m.sup.-1. This performance is believed to be due to a combination of the negative electron affinity of the (111) facets of diamond and the high density of localised, accidental graphite inclusions (Xu, Latham and Tzeng: Electron. Lett. 1993, 29, pp 1596-159).
Coatings with a high diamond content can now be grown on room temperature substrates using laser ablation and ion beam techniques. However, all such processes utilise expensive capital equipment.
S I Diamond in the USA has described a field electron emission display (FED) that uses as the electron source a material that it calls Amorphic Diamond. The diamond coating technology is licensed from the University of Texas. The material is produced by laser ablation of graphite onto a substrate. FIG. 2 shows the conceptual arrangement in such a display. A substrate 4 has conducting strips 5 with Amorphic diamond emitting patches 6. A front plate 8 has transparent conducting tracks 7 with an applied phosphor pattern (not shown). Pixels are addressed using a crossbar approach. Negative going waveforms 9 are applied to the conductive strips 5 and positive going waveforms are applied to conductive strips 7. The use of positive and negative going waveforms both reduces the peak voltage rating for the semiconductors in the drive electronics and ensures that adjacent pixels are not excited. The device is sealed around the perimeter and evacuated.
Turning now to Composite Field Emitters, current understanding of field electron emission from flat metal surfaces shows that active sites are either metal-insulator-vacuum (MIV) structures formed by embedded dielectric particles or conducting flakes sitting on the surface oxide of the metal. In both cases, the current comes from a hot electron process that accelerates the electrons resulting in quasi-thermionic emission. This is described in the scientific literature (e.g. Latham, High Voltage Vacuum Insulation, Academic Press 1995)
In 1988 (S Bajic and R V Latham, Journal of Physics D Applied Physics, vol. 21 (1988) 200-204), a material that made practical use of the above mechanism was described. The composite material creates a high density of metal-insulator-metal-insulator-vacuum (MIMIV) emitting sites. The composite had conducting particles dispersed in an epoxy resin. The coating was applied to the surface by standard spin coating techniques.
The emission process is believed to occur as follows. Initially the epoxy resin forms a blocking contact between the particles and the substrate. The voltage of a particle will rise to the potential of the highest equipotential it probes--this has been called the antenna effect. At a certain applied voltage, this will be high enough to create an electro-formed conducting channel between the particle and the substrate. The potential of the particle then flips rapidly towards that of the cathode. The residual charge above the particle then produces a high electric field which creates a second electro-formed channel and an associated MIV hot electron emission site. After this switch-on process, reversible field emitted currents can be drawn from the site. The current density/electric field performance of this material is equivalent to broad area diamond emitters produced by the much more expensive laser ablation process.
Bajic and Latham worked with resin-carbon composites. Although they considered the use of alternative materials, these were always composites with resin (supra and Inst Phys Conf Ser No. 99; Sectzon 4-pp 101-104, 1989). Epoxy resins provided materials that were convenient to work with, particularly in view of their adhesive properties making it convenient to place and hold particles where desired, in composite or layered structures. However, materials such as those produced by Bajic and Latham have tended to have poor stability, and not to work satisfactorily in sealed-off vacuum devices.